Hi Chaitali,
U need to be more specific;
What kind of S7 are you looking for ? S7-200, S7-300 or S7-400.
Do you also need to buy the software Microwin for S7-200 and Step7 -300 and S7-400.
What kind of I/O do you need? digital in/outputs, analog in/outputs.
Is there a network required Profibus, Industrial Ethernet enz.
What other kind of PLC had you in mind for your price comparison.
It's not possible to answer your question unless we can receive this kind of information.
Rudi